The present invention relates to a circuit breaker, such as an autobreaker, applied to the protection of an electric motor.
Manufacturers of such circuit breakers, which are main components of low-voltage distribution devices, define the products with the construction described below as a standard and produce variations thereof to meet users"" various needs.
The construction of the standard circuit breaker is shown in FIGS. 5, 6(a), and 6(b), using an autobreaker by way of example. In FIG. 5, reference numeral 1 denotes a circuit breaker case (resin mold case), 2 is a power-supply-side main circuit terminal, 3 is a load-side main circuit terminal, 4 is a handle for opening-and-closing operations, 5 is an adjustment dial for adjusting a rated current, with the adjustment dial facing a cover 1a of the case 1, 6 is a test trip slot into which a screwdriver or the like is inserted from the outside for a trip test, and 7 is a name plate. The case 1 has a main circuit breaking section 8, which is formed of a movable contact shoe 8a, a fixed contact shoe 8b, and an arc-extinguishing chamber 8c; a toggle-link-type opening and closing mechanism section 9 for driving the movable contact shoe 8a of the breaking section 8 between an open position and a closed position; a thermal overload/open-phase tripping device 10 corresponding to each phase; and an electromagnetic instantaneous tripping device 11. These components are arranged as illustrated in FIGS. 6(a) and 6(b).
In this case, the thermal overload/open-phase tripping device 10 and the electromagnetic instantaneous tripping device 11 are integrally assembled together for each phase, to constitute a trip unit. The thermal overload/open-phase tripping device 10 is formed of a combination of a heater-mounted main bimetal 12 connected to each phase of the main circuit, a differential shifter mechanism 13 linked with a main bimetal operating end (upper end) for each phase so as to interconnect with the bimetal, a temperature-compensating bimetal 14 for linking an output end of the differential shifter mechanism 13 with a latch receiver incorporated into the opening and closing mechanism section 9, with the temperature-compensating bimetal 14 also being used as a tripping lever, and the adjustment dial 5 described previously.
Further, the differential shifter mechanism 13 is formed of a combination of a sliding push shifter 15 and a sliding pull shifter 16, which are positioned along the main bimetals 12 for the respective phases on the respective sides thereof, and are guided and supported in a groove in an interphase partition wall 1b of the case 1, with an output lever 17 extending over the push shifter 15 and the pull shifter 16 and being pivotally coupled with pins. The push shifter 15 and pull shifter 16 have L-shaped arms 15a and 16a, respectively, which project toward the main bimetal 12 for each phase, so that in an assembled position, the tips of the arms are located opposite to the respective surfaces of the main bimetal 12 so as to sandwich the main bimetal therebetween. Furthermore, the adjustment dial 5 described above has a groove 5a formed in a top surface thereof into which a screwdriver or the like is inserted for operation. A rated current value is printed around the periphery of a dial hole opened in the case cover 1a so as to correspond to an arrow printed on the top surface of the dial 5.
The operation of the thermal overload/open-phase tripping device 10 is well known. When an overload current continuously flows through the main circuit, the main bimetal 12 is bent correspondingly in a predetermined direction under heat by the heater, and the push shifter 15 and pull shifter 16 of the differential shifter mechanism 13 are displaced in the direction indicated by an arrow in FIG. 6(a) so as to follow the bending of the bimetal. The output lever 17 then pushes the tip of the temperature compensating bimetal 14. This causes the temperature compensating bimetal 14 to rotate clockwise to push the latch receiver into its released position, and in synchronism with this movement, the opening and closing mechanism section 9 performs a trip operation to open the movable contact shoe 8a of the breaking section 8, interrupting the current flowing through the main circuit. If an open phase occurs, the push shifter 15 and pull shifter 16 of the differential shifter mechanism 13 operate in a differential manner to cause the output lever 17 to rotate counterclockwise around the pin for coupling with the pull shifter in order to push the temperature compensating bimetal 14, thereby causing the circuit breaker to perform a trip operation as described above.
On the other hand, the electromagnetic instantaneous tripping device 11 is formed of a trip coil 11a, which is commonly referred to as an xe2x80x9cinstant coilxe2x80x9d, a yoke 11b, a plunger 11c, and a tripping lever 11d that follows the operation of the plunger 11c. The trip coil 11a and the heater-mounted main bimetal 12 are connected together in series, and are interposed between and connected to the load-side main circuit terminal 3 and the fixed contact shoe 8a of the breaking section 8. When an overcurrent, such as short circuit current, flows through the main circuit, the plunger 11c performs a suctioning operation to cause the tripping lever lid to depress a tripping plate incorporated in the opening and closing mechanism section 9, thereby driving the latch receiver described above into its released position in order to cause the circuit breaker to instantaneously perform a trip operation.
If the opening and closing mechanism section 9 undergoes a trip test in a non-conductive state, a screwdriver or the like is inserted into the test trip slot 6, shown in FIG. 5, from the outside in order to move the output lever 17 (which has a projection to be caught by a tip of the screwdriver) of the differential shifter mechanism 13 (see FIGS. 6(a) and 6(b)). Then, as in the tripping operation for the overload current, the latch receiver of the opening and closing mechanism section 9 moves to its released position via the temperature compensating bimetal 14 to perform a trip operation.
When a standard circuit breaker provided with the thermal overload/open-phase tripping device 10 and the electromagnetic instantaneous tripping device 11 as standard equipments is applied to a distribution circuit using as a load an electric motor requiring an extended period for start-up, the start current and time for the electric motor can not be coordinated with the overload protection characteristics of the circuit breaker. Consequently, while the electric motor is being started up, the thermal overload/open-phase tripping device 10 may operate to cause the circuit breaker to perform a trip operation.
Thus, a feeding circuit using as a load an electric motor requiring a particularly long time for start-up may employ a circuit breaker comprising the standard circuit breaker described above, which does not perform the overload/open-phase tripping function in order to prevent the circuit breaker from inadvertently performing a trip operation while the electric motor is being started up. Instead, a thermal relay is connected to the load side of the circuit breaker to protect the electric motor from overload.
Further, in a variation of a circuit breaker that is adapted for the above application conventionally, the differential shifter mechanism 13, the temperature compensating bimetal 14, and the adjustment dial 5 are removed from the standard circuit breaker shown in FIGS. 6(a) and 6(b) in order to disable the overload/open-phase tripping function, and the case cover 1a, shown in FIG. 5, is modified so that the dial hole and test trip slot therein are blocked.
However, the conventional instantaneous-tripping circuit breaker, which is provided as the variation of the circuit breaker by removing accessory parts of the overload/open-phase tripping device from the standard circuit breaker and replacing the case cover thereof with a modified one, entails economic and functional problems, as stated below:
(1) As the case cover differs from that of the standard circuit breaker, a new mold must be provided to produce the case cover, thus increasing a cost.
(2) As the differential shifter mechanism 13 of the overload/open-phase tripping device 10 is not incorporated, the test trip function by using the differential shifter mechanism is eliminated.
(3) Furthermore, as the heater-mounted main bimetal 12 of the overload/open-phase tripping device 10 is left connected to the main circuit in series with the trip coil of the electromagnetic instantaneous tripping device, a circuit breaker with a high rated current can not withstand the required amount of overload current for the instantaneous tripping method. That is, the heater of the main bimetal 12 is selected to have a resistance value that allows an overload/open phase in the main circuit to be detected in order to generate a sufficient amount of heat to bend the bimetal. Thus, if a high overload current continuously flows, as in the application of the conventional circuit breaker to the load circuit of a large electric motor requiring an extended period for startup, the amount of heat generated by the heater increases, possibly resulting in melting and breaking the heater.
The present invention has been made in view of these points, and it is an object thereof to provide an instantaneous-tripping circuit breaker that disables the overload/open-phase tripping function while maintaining the test trip function without significant increase in cost, simply by removing a part of the thermal overload/open-phase tripping device from the standard circuit breaker provided with the thermal overload/open-phase tripping device and the electromagnetic instantaneous tripping device as standard equipments and replacing it with another part, with the instantaneous-tripping circuit breaker being able to be safely applied to an electric motor requiring an extended period for start-up.
To attain this object, the present invention provides an instantaneous-tripping circuit breaker. In this case, a standard circuit breaker comprises a breaking section, an opening and closing mechanism section, a thermal overload/open-phase tripping device, and an electromagnetic instantaneous tripping device, all installed in a breaker case. The thermal overload/open-phase tripping device includes an assembly of a heater-mounted main bimetal connected to each phase of a main circuit, a differential shifter mechanism moved together with the main bimetal, a temperature compensating bimetal for linking an output end of the differential shifter mechanism with a latch receiver of the opening and closing mechanism section, with the temperature compensating bimetal also being used as a tripping lever. An adjustment dial is also provided for making adjustments of the rated current. In the first aspect of the invention, the heater-mounted main bimetal is omitted from a trip unit of the standard circuit breaker, and a trip coil of the electromagnetic instantaneous tripping device is connected to the main circuit via a connection conductor having a lower resistance than the heater of the main bimetal.
As described above, the heater-mounted main bimetal is removed from the thermal overload/open-phase tripping device installed in the standard circuit breaker, and the connection conductor of a lower resistance is used in place of the heater to connect the trip coil of the electromagnetic instantaneous tripping device to the main circuit. Consequently, the required amount of the overload current for the instantaneous-tripping circuit breaker can be obtained to make the circuit breaker more reliable.
In the construction of the first aspect, in place of the differential shifter mechanism installed in the standard circuit breaker, a test-tripping part is assembled in the breaker case, and an output lever supported on the test-tripping part is located opposite to the temperature compensating bimetal, and is operated from the outside of the case to conduct a trip test on the breaker (second aspect of the invention). With this construction, by simply replacing the differential shifter mechanism of the overload/open-phase tripping device with the simple test-tripping part, the temperature compensating bimetal can be used to provide a trip test function similar to that provided by the standard circuit breaker.
Moreover, in the construction of the first aspect, the adjustment dial for the rated current is replaced with a dummy dial having no adjustment function and facing a dial hole in the breaker case (third aspect of the invention). Thus, the case cover of the standard circuit breaker can be applied directly to the breaker case to allow parts to be shared. Further, as no adjustment groove is formed in the top surface of the dummy dial, it can be visually determined that this circuit breaker is based on an instantaneous tripping type without any overload/open-phase tripping function.